Corundum. Credit: Mineral Information Institute.
Corundum is a translucent to transparent crystalline (rhombohedral) form of aluminum oxide (alpha-Al2O3) which comes in many different hues. It is found in igneous and metamorphic rocks, occurring as pyramidal or prismatic crystals in the rhombohedral class and as granular masses. The corundum structure has close-packed oxygen with aluminum in octahedral holes. Mineral varieties of corundum include ruby, sapphire, and emery. Artificial corundum, or beta-alumina, is made by calcining bauxite (see calcination).
Corundum is the hardest natural substance known after diamond and is also chemically inert and resistant to corrosion. It is used extensively as an abrasive and in bearings (for example, in motors and watches). Hardness 9, relative density 4.